The multiconfiguration self-consistent field (MCSCF) method offers the most general approach to the computation of chemical reactions and multiple electronic states. This review discusses the design of MCSCF wavefunctions for treating these problems and the interpretation of the resulting orbitals and configurations. In particular, localized orbitals are convenient both for selection of the appropriate active space and for understanding the computed results. The computational procedures for optimizing these wavefunctions and the techniques for recovery of dynamical correlation energy are reviewed.

The construction and interpretation of MCSCF wavefunctions.

An Introduction to Coupled Cluster Theory for Computational Chemists

Efficient computer implementation of the renormalized coupled-cluster methods: The R-CCSD[T], R-CCSD(T), CR-CCSD[T], and CR-CCSD(T) approaches

In this paper we present a comprehensive account of a manifestly size-consistent coupled cluster formalism for a specific state, which is based on a reference function composed of determinants spanning a complete active space (CAS). The method treats all the reference determinants on the same footing and is hence expected to provide uniform description over a wide range of molecular geometry. The combining coefficients are determined by diagonalizing an effective operator in the CAS and are thus completely flexible, not constrained to preassigned values. A separate exponential-type excitation operator is invoked to induce excitations to all the virtual functions from each reference determinant. The linear dependence inherent in this choice of cluster operators is eliminated by invoking suitable sufficiency conditions, which in a transparent manner leads to manifest size extensivity. The use of a CAS also guarantees size consistency. We also discuss the relation of our method with the extant state-specific...

A size-consistent state-specific multireference coupled cluster theory: Formal developments and molecular applications

This paper is the first in a series of papers on the new approach to the many-electron correlation problem, termed the method of moments of coupled-cluster equations (MMCC). A hierarchy of MMCC approximations, including the renormalized and completely renormalized CCSD[T], CCSD(T), CCSD(TQ), and CCSDT(Q) methods, which can be viewed as generalizations of the well-known perturbative coupled-cluster CCSD[T], CCSD(T), CCSD(TQf), and CCSDT(Qf) schemes, is introduced. In this initial study, an emphasis is placed on the ability of the MMCC approach to describe bond breaking and large effects due to connected triples and quadruples by modifying the standard noniterative CC approaches, such as the popular CCSD(T) method. The performance of selected MMCC approaches, including the renormalized and completely renormalized CCSD[T], CCSD(T), and CCSD(TQ) schemes, is illustrated by the results of pilot calculations for the HF and H2O molecules.

The method of moments of coupled-cluster equations and the renormalized CCSD[T], CCSD(T), CCSD(TQ), and CCSDT(Q) approaches

The recently developed state‐selective (SS) multi‐reference coupled‐cluster (CC) method involving all singly and doubly, and semi‐internal triply excited clusters from the formal reference configuration [SSCCSD(T) approach] is tested in the calculation of the potential energy surface (PES) of the HF molecule. Both double zeta and double zeta plus polarization basis sets are employed and a few different choices of active space are considered. The SSCCSD(T) method provides an accurate description of the entire PES at low cost even for the bond breaking region, contrary to the results obtained with the perturbative single‐reference CCSD(T) method or various limited configuration interaction approaches. This is the first application of the new SSCC code, which uses an improved computational strategy for handling the semi‐internal triexcited clusters. Details of this new implementation of the SSCCSD(T) method are discussed.

Improved computational strategy for the state‐selective coupled‐cluster theory with semi‐internal triexcited clusters: Potential energy surface of the HF molecule

This presentation will provide a comprehensive account of the recent developments in the the state-selective coupled cluster method for quasi-degenerate electronic states. We will mention some prev...

The state-selective coupled cluster method for quasi-degenerate electronic states

In this work, we examine the efficiency of the recently developed [P. Piecuch et al., J. Chem. Phys. 99, 6732 (1993)] state‐selective (SS) multi‐reference (MR) coupled‐cluster (CC) method for calculation of molecular properties. In our earlier papers, we demonstrated that the SSMRCC method with inclusion of single, double, and internal and semi‐internal triple excitations [SSCCSD(T) approach] is capable of providing an accurate description of the ground‐state potential energy surfaces. In this paper, we present the dipole moment and polarizability values of the HF molecule at equilibrium and stretched geometries calculated using finite field technique and SSCCSD(T) ansatz. The calculations use double zeta quality basis sets with and without polarization functions. Molecular orbital basis sets include both relaxed and nonrelaxed orbitals.

State‐selective multireference coupled‐cluster theory: In pursuit of property calculation

In this paper, we report the use of extended coupled cluster functional of Arponen, Bishop, and co‐workers to implement a stationary biorthogonal response approach. The objective of this is to calculate nonlinear molecular properties like hyperpolarizability, etc. in a more convenient way.

Keya Basu Ghose

Papers

Improved computational strategy for the state‐selective coupled‐cluster theory with semi‐internal triexcited clusters: Potential energy surface of the HF molecule

The state-selective coupled cluster method for quasi-degenerate electronic states

State‐selective multireference coupled‐cluster theory: In pursuit of property calculation

Nonlinear molecular properties using biorthogonal response approach

Fock-space multireference coupled-cluster theory. fourth-order corrections to the ionization potential☆